It is well known to treat subterranean formations in order to plug areas of high permeability. This leads to a better injection profile in connection with the use of injection wells and to overall improvement in flow profiles and sweep efficiencies. A number of methods of treating formations of high permeability have been employed or suggested, including the use of various cements and gels for sealing the open structure of the formation. A particularly useful gel system is described in U.S. Pat. No. 4,683,949, wherein a gel is prepared by forming a gelation solution above ground from a water-soluble acrylamide polymer and a cross-linking agent. The solution is then injected into the formation to be treated in at least partially gelled form to inhibit propagation into regions where no treatment is desired. This was an improvement over systems which required sequential injection of the gel components into a formation, a practice that often led to an inability to achieve complete mixing and gelation in the formation.
Even with the improved system referred to, difficulty is encountered in employing gel to plug large fissures because the larger masses of gels required often lack the necessary strength to resist the pressures to which they are exposed. In addition to the gel being required to seal relatively large fractures, for example, 1/8 inch or more in width, it must be possible to pump the gel or gelation solution for quite long distances, often in the range of 4000 to 5000 feet. It is suggested in the patent referred to that inert fillers such as fine rock particles or glass beads can be added to the gelation solution to reinforce the gel network structure. Such measures are not sufficient, however, to strengthen the gel structure to the extent necessary to bridge and remain in place in the openings encountered in highly permeable formations.
U.S. Pat. No. 3,701,384 discloses a method of sealing thief zones in a subterranean formation by plugging pores with a solid material. A slurry of finely divided inorganic solids is injected into the formation together with an aqueous colloidal dispersion of a water-insoluble metal hydroxide in a dilute aqueous solution of a high-molecular-weight organic polymeric polyelectrolyte. The preferred polymer solution contains between about 0.01 and about 0.2 percent by weight of high molecular weight polyacrylamide or hydrolyzed polyacrylamide. At these concentrations, the dissolved polymer causes the suspended solids to flocculate, thereby blocking pores in the formation. The tested inorganic solids which interacted with the polymer solution to form strong solids included finely ground asbestos fibers and magnesium oxide. However, asbestos is undesirable for use today, due to its carcinogenicity.
Another approach taken by the prior art is to pump a slurry containing a mixture of flexible fibers and a bonding agent into highly permeable portions of a formation interval. An agent which precipitates or gels the bonding agent is then injected into the interval. The goal of the method is to build up a filter cake of fibers on the permeable formation as a result of the fibers being deposited out of the slurry as the slurry flows through, the permeable formation, and then bond the fibers of the filter cake in place. Examples of such a method are disclosed in U.S. Pat. Nos. 3,593,798, 3,949,811 and 3,462,958.
Larger fissures are bridged according to the disclosure of U.S. Pat. No. 2,708,973 by setting fibrous plants in place in the fissure, after which cement is added, thereby building on the framework of the plants. While such a method can bridge larger gaps, the process is impractical for use in deep formations that extend over a large area.
U.S. Pat. No. 3,374,834 discloses a method of stabilizing earth formations by injecting an aqueous solution of gelling material which contains finely divided inert solids and needle-like crystals of silicate materials which act as a suspending agent to prevent premature settling out of the solids. The resulting gel does not, however, provide the desired combination of strength, economy, ease of mixing and ability to be readily introduced into a formation.